Conventionally, the following printer is known in patents (e.g., Japanese Patent Laid-Open No. 10-308868). The printer can add the same identification information to an image data and a printed product, and can make a device connected to the printers store temporarily the image data When some printers get an instruction to copy the printed product, those printers search for the stored image data on the basis of the identification information of the printed product, and print the stored image (see, e.g., Japanese Patent Laid-Open No. 10-308868). According to this technique, even if the image quality of the printed product-printed by the printer is degraded, users can get a high-quality printed product.
Recently, the awareness of security is growing, and a method of managing an output-product is becoming increasingly important. So, there is known a technique called “copy-forgery-inhibited-pattern image-print” which utilizes the limit of the reproducibility of the copying machine (scanner). According to this technique, a content image is synthesized with a copy-forgery-inhibited-pattern image to which a character string such as “COPY” is added, and the synthesized image is formed on output-paper. When the output-product is copied, the added character string such as “COPY” appears on the copy (see, e.g., Japanese Patent Laid-Open No. 2001-324898).
A person who obtains a copy of the output-product having a copy-forgery-inhibited-pattern image and content image may refrain from leaking the information when seeing the character string, or may recognize that the information is confidential and refrain from bringing the information out the company. The copy-forgery-inhibited-pattern image is said to be a technique of inhibiting leakage of information by appealing to the human psychology, and a technique of ensuring security to a certain degree.
A copy-forgery-inhibited-pattern image will be explained in detail. A character string such as “COPY” is added in a copy-forgery-inhibited-pattern image so that a person cannot recognize the character string at a glance. However, the added character string appears on a copy obtained by copying the copy-forgery-inhibited-pattern image. Thus, a document (to be referred to as an original document hereinafter which is formed on output-paper by synthesizing the copy-forgery-inhibited-pattern image and document image can be easily discriminated from a copy of the original document. Since the character string such as “COPY” appears on the copy, a person who has copied the document may hesitate to use the copy.
The copy-forgery-inhibited-pattern image is made up of two areas. One area “remains” on a copy upon copying, whereas the other area “disappears” (or “becomes lighter in color than the remaining area”). The reflection densities in these two areas are almost equal to each other on the original document, and thus an added character string such as “C” cannot be recognized by the human eye. In this case, “remain” means that an image on an original document is faithfully reproduced on a copy. “Disappear” means that an image on an original document is not reproduced on a copy. Note that the reflection density is measured by a reflection densitometer.
In the following description, an area which “remains” on a copy will be referred to as a “latent image portion”, and an area which “disappears” (or “becomes lighter in color than the remaining area”) will be referred to as a “background portion”.
FIG. 21 is showing the state of dots on a copy-forgery-inhibited-pattern image. In FIG. 21, an area where dots are clustered and laid out is a latent image portion, and an area where dots are dispersed and laid out is a background portion. Dots in these two areas are generated by different halftone screening processes or different dithering processes. For example, dots at the latent image portion are generated by a halftone screening process at a low LPI, while dots at the background portion are generated by a halftone screening process at a high LPI. Alternatively, dots at the latent image portion are generated using a clustered-dot dither matrix, whereas dots at the background portion are generated using a dispersed-dot dither matrix.
The reproducibility of a copying machine is limited. This reproducibility depends on the input and output resolutions of the copying machine. When dots at the latent image portion are formed larger than the dot that a copying machine can reproduce and dots at the background portion are formed smaller than the dot that a copying machine can reproduce, dots at the latent image portion are generally reproduced on a copy, but dots at the background portion are hardly reproduced. As a result, the latent image portion is reproduced darker than the background portion on the copy. A phenomenon in which an added character string looks popping up because the latent image portion is reproduced darker than the background portion on a copy will be referred to as “visualization”.
Images 2101 and 2102 in FIG. 21 show visualization. FIG. 21 conceptually illustrates visualization in which clustered dots (large dots) are reproduced on a copy and dispersed dots (small dots) are not accurately reproduced on the copy.
Note that the copy-forgery-inhibited-pattern image is not limited to the above layout. For example, the “COPY” area can be a background portion, and the other area except the “Copy” area can be a latent-potion. In this example, a character string such as “COPY” is lighter than the other area in a copy.
As described above, when copying of a printed product with identification information is designated, a corresponding image can be read out from a storage device to output a high-quality copy. However, if the person distributes the high-quality copy of the image to the other person, the other person can copy the high-quality copy. As a result, the possibility of the information leak is high if the copy of the high-quality copy is distributed. That is a security problem.